CHAPTER XX 



INDIVIDUATION— THE FORMATION OF PATTERN AND 



SHAPE 



DIFFERENTIATION does not consist merely in the production of new 

 'substances' — be they simple ones, such as pigments, or complex, such as 

 the various types of tissue. We also have to consider the arrangement of 

 these substances into definite relative positions, and, usually, the moulding 

 of them into characteristic shapes. Development, in fact, produces not 

 merely tissues but organs. It is to this type of phenomenon that the name 

 'individuation' has been given. It has, as has been impHed above, two 

 rather different aspects. On the one hand there is the question of the 

 spatial distribution of the different substances. For instance, within the 

 sheet of developing mesoderm a notochord develops in the dorsal mid- 

 line, flanked on either side by somites, with the nephric mesoderm more 

 laterally again and side-plate mesoderm outside that. We have earlier 

 (p. 12) used the term 'regionalisation' to refer to the appearance of 

 different parts within an originally uniform expanse of tissue. This is one 

 of the aspects of individuation and before considering it further we should 

 remind ourselves that regionalisation normally takes place so as to produce 

 definite patterns of arrangement of the different parts. It is not adequate 

 to picture it merely as a process by which a number of intermingled 

 entities become sorted out into heaps of like components ; we must add 

 the fact that the heaps are mutually arranged in orderly patterns. 



The other aspect of individuation is the formation of three-dimensional 

 structures. For instance, the hollow sphere of the blastula undergoes 

 the process of gastrulation and thus acquires a new and definite con- 

 figuration; or again, a neural plate rolls up into a neural tube, which is 

 characterised by the well-defined swellings of the brain vesicles, etc. AH 

 such processes are 'morphogenesis' in the strict sense, since that word 

 really means the development of shape. The shapes of organs and of the 

 body as a whole continue to change throughout most of life owing to the 

 unequal growth of different parts. Such processes of relative growth have 

 been considered in Chapter XIII; they may be considered as secondary 

 morphogenesis. What we shall be concerned with in this chapter are the 

 processes of primary morphogenesis, by which the original shape of the 

 organ rudiments is first brought into being. (The distinction between 

 these two categories of primary and secondary morphogenesis is not 



415 



